The USB adapter route might work, since we're only talking about hardware adaptation. It'll be slow at USB 1.1, but it may work acceptably. This would be an older iMac with no Firewire and no PCI slots, right?
I don't think I'd spend the money on the adapter thing, however. It's likely to cost at least $100. For $250 or so you should be able to buy a used iMac DV which has the Firewire connection. It's more money but (a) you know it'll work and (b) you can sell the old iMac to someone who doesn't care about iPod for the same price, making it cost-neutral.